Perennial biomass crop establishment, community characteristics, and productivity in the upper US Midwest: Effects of cropping systems seed mixtures and biochar applications

Native perennial plants have potential as bioenergy feedstocks, but their use is currently limited by relatively long establishment times and low biomass yields. Some research suggests that incorporating plant species diversity and applying biochar as a soil amendment might alleviate these limitations by creating a more resilient crop and soil system. The objective of this research was to investigate how 1) seeded plant diversity and 2) biochar soil amendments interact to affect the establishment, yield, and plant species composition of biomass cropping systems during the first four years of growth on productive soils. We measured species emergence, cover, peak and post-frost biomass, and biomass composition for three biomass cropping systems seed mixtures – a switchgrass monoculture, a three-species grass mixture, and a highly diverse mixture of grasses and forbs – either with or without application of a mixed wood gasification biochar (9.3 Mg ha⁻¹). We found that seed mixture had significant effects on nearly every variable measured, with switchgrass monocultures outperforming the two more diverse mixtures by the third year of the experiment (12.0 Mg ha⁻¹ in switchgrass, 8.7 Mg ha⁻¹ in low diversity plots, and 3.9 Mg ha⁻¹ in high diversity plots), despite an initial switchgrass establishment failure. The high diversity plots exhibited poor sown species establishment in the first year due to high weed pressure in a drought year, but continued to improve over time. Biochar application had no consistent effect on plant biomass or community traits, and significantly affected only two community traits, light transmittance and leaf area index. Our results suggest that on productive soils perennial bioenergy productivity may be achieved through selection of one or a few high-yielding grass species, with little or no effect of biochar applications on perennial biomass crop establishment, diversity, or productivity.